Precipitated silica is conventionally produced from sodium silicate solutions and acids, most often sulfuric acid. An alternative raw material for precipitated silica is olivine, a natural magnesium iron silicate available in large quantities at many locations in the world. Olivine is easily soluble in acid and has been considered as a raw material for magnesium chemicals and silica. The raw mined olivine can contain up to 5-8 wt % of accessory minerals (e.g., pyroxene, spinel, and chlorite) that are poorly soluble in acid, and that will contaminate the precipitated silica unless proper measures are taken. Olerud (U.S. Pat. No. 5,780,005) describes a process for production of silica from olivine. Olerud discloses a process including pretreatment of the olivine in order to remove from it most of the accessory minerals that might otherwise contaminate precipitated silica produced from the olivine. The process by Olerud also includes features for controlling the specific surface area of the produced silica. It is demonstrated that acid strength, temperature, and leaching time all have an effect on the specific surface area of the silica.
Another process for the production of active silica from natural silicates has been described by Maslo (UK patent application GB 2 078703 A). The process focuses on the production of silica from serpentine and the content of impurities in the silica obtained is relatively high. WO02/48036 A1 describes a process for the production of silica from olivine. This process is based on the sulfatisation of olivine with concentrated sulfuric acid at approx. 250° C., followed by leaching in water to give precipitated silica, which is then subjected to further purification steps.
Jas (U.S. Pat. No. 4,537,699) discloses a process for providing stable sprayable suspensions of precipitated silica, with a relatively high solids content, and a pH value greater than 3.5-4, by adding an aluminate compound such as sodium aluminate to precipitated silica.
New efficient and economical methods to produce precipitated silica from olivine would be greatly appreciated, in particular methods to produce silica with high surface area and a high degree of purity.